1. Field of the Invention
The present invention relates to ring LAN (Local Area Network) systems, and more particularly to multi-ring LAN systems suitable for high-speed data transmission and node devices constituting such system.
2. Description of the Related Art
Conventionally, a bus type LAN having a transmission rate, for example, of 150 Mbps, is described in LEEE COMMUNICATION MAGAZINE, Vol. 26, No. 4 (1988), pp. 20-28. A ring LAN having a transmission rate of 200 Mbps in which a central supervisiory node in a slotted ring stores received information in a buffer such that a loop round-trip delay is an integer times the length of a frame is described in GLOBECOM', 85, 1-4. This reference further describes a method of operating all the respective nodes using a clock extracted from a received signal. A conventional slotted ring accessing system in which a node which acquires a right to access and uses a slot necessarily empties out the slot used after it has made a round trip through the ring is described in "IEEE, TRANSACTION COMMUNICATIONS", COM-29 (1981), p. 1466.
An ATM (Asynchronous Transfer Node) cell using a conventional ring LAN is described in Institute of Electronics, Information and Communication Engineers of Japan, "KOKAN SYSTEM KENKYUKAI SHIRYO" (SWITCHING SYSTEM RESEARCH SOCIETY MATERIAL) SSE 88-93.
Conventionally, the transmission rate in a high-speed ring LAN having a transmission rate higher than 100 Mbps is an integer times 100 Mbps as a reference. A public network employs a transmission rate of 155.52 Mbps which is a standard transmission rate according to CCITT (Consultative Committee in International Telegraphy and Telephony), so that if a LAN and a public network are interconnected, a buffer to cause both the transmission rates to match each other is required, thereby causing the connection device to be large-sized.
In the conventional slotted ring, the central supervisory node buffers received information such that the ring round-trip delay of a communication frame is an integer times the length of the synchronous frame in consideration of the accommodation of synchronous information exchanged on a time-divisional basis. Thus, the transmission delay of the signal would increase to thereby reduce the throughput between user devices.
In the conventional access system in which each node which has acquired a right to access necessarily releases a slot used by that node in the next round trip, the transfer efficiency is reduced in the transfer of data longer than an information transfer area in a cell in te ring under low traffic conditions. For example, if (1) the frame period is 125 .mu.sec, (2) the ring round-trip delay is a time corresponding to one frame, and (3) the cell length is 53 bytes (including 48 bytes of the information transfer area), the transfer rate is 155 Mbps and 44 cells are present on the ring. At this time, assume that the transfer of data of 4,000 bytes occurs at a node. It is necessary to acquire a right to access at least three times before data transfer is completed in the conventional system, and an access waiting time is required which includes at least a time required for three ring round-trips. One of the three round-trips is spent for the round-trip of an empty slot.
Since in the conventional LAN there is only one kind of frame in the transfer path, and a node processes the frames in a serial manner, it is necessary to increase the processing rate of all the nodes in order to increase the transmission rate. Since each node uses a clock extracted from received data when the data is transmitted, there is a limit to the number of nodes which may be connected to a ring due to accumulation of jitters.